strings.h: Fix issues with escape string functions.

Fixes for issues with the ASTERISK-24934 patch.

* Fixed ast_escape_alloc() and ast_escape_c_alloc() if the s parameter is
an empty string.  If it were an empty string the functions returned NULL
as if there were a memory allocation failure.  This failure caused the AMI
VarSet event to not get posted if the new value was an empty string.

* Fixed dest buffer overwrite potential in ast_escape() and
ast_escape_c().  If the dest buffer size is smaller than the space needed
by the escaped s parameter string then the dest buffer would be written
beyond the end by the nul string terminator.  The num parameter was really
the dest buffer size parameter so I renamed it to size.

* Made nul terminate the dest buffer if the source string parameter s was
an empty string in ast_escape() and ast_escape_c().

* Updated ast_escape() and ast_escape_c() doxygen function description
comments to reflect reality.

* Added some more unit test cases to /main/strings/escape to cover the
empty source string issues.

ASTERISK-25255 #close
Reported by: Richard Mudgett

Change-Id: Id77fc704600ebcce81615c1200296f74de254104

1 files changed, 10 insertions, 9 deletions

diff --git a/include/asterisk/strings.h b/include/asterisk/strings.h
index d361293d0..af5ae6c55 100644
--- a/include/asterisk/strings.h
+++ b/include/asterisk/strings.h
@@ -312,32 +312,33 @@ char *ast_unescape_c(char *s);
 /*!
  * \brief Escape the 'to_escape' characters in the given string.
  *
- * \note The given output buffer has to have enough memory allocated to store the
- *       original string plus any escaped values.
+ * \note The given output buffer will contain a truncated escaped
+ * version of the source string if the given buffer is not large
+ * enough.
  *
  * \param dest the escaped string
  * \param s the source string to escape
- * \param num number of characters to be copied from the source
+ * \param size The size of the destination buffer
  * \param to_escape an array of characters to escape
  *
  * \return Pointer to the destination.
  */
-char* ast_escape(char *dest, const char *s, size_t num, const char *to_escape);
+char *ast_escape(char *dest, const char *s, size_t size, const char *to_escape);
 
 /*!
  * \brief Escape standard 'C' sequences in the given string.
  *
- * \note The given output buffer has to have enough memory allocated to store the
- *       original string plus any escaped values.
+ * \note The given output buffer will contain a truncated escaped
+ * version of the source string if the given buffer is not large
+ * enough.
  *
  * \param dest the escaped string
  * \param s the source string to escape
- * \param num number of characters to be copied from the source
- * \param to_escape an array of characters to escape
+ * \param size The size of the destination buffer
  *
  * \return Pointer to the escaped string.
  */
-char* ast_escape_c(char *dest, const char *s, size_t num);
+char *ast_escape_c(char *dest, const char *s, size_t size);
 
 /*!
  * \brief Escape the 'to_escape' characters in the given string.